1. Field of the Invention
The present invention relates to a machine such as a machine tool and various kinds of industrial machines, in which a movable unit is controllably driven by a servo motor, and particularly, to a machine having a function of detecting the collision of a movable unit against other members.
2. Description of the Related Art
There has been known a machine, in which the collision of a movable unit to be driven by a servo motor against other members such as a workpiece or a jig is detected based on an estimated disturbance load torque obtained by estimating a disturbance torque by means of an observer or a drive current value of the servo motor in a machine tool and various kinds of industrial machines. Some examples will be described below.
(a) An output torque from a servo motor is detected, and then, an inertia load is corrected according to the detected torque to obtain an external load. Thereafter, the collision of a movable unit against other members is detected by comparing the obtained external load with a level of a set overload. The level of the overload to be compared with the external load is switched to a high level of the set overload when a movement command is a low speed movement command (i.e., a machining command): in contrast, it is switched to a low level of the set overload when the movement command is a fast feed command (see Japanese Patent Application Laid-open No. 10-143216).
(b) A disturbance torque value is estimated by a disturbance torque observer, and then, an estimated disturbance power value is obtained by multiplying the estimated disturbance torque value by a rotational speed of a motor. It is determined that there is a collision in the case where the estimated disturbance power value exceeds an allowable disturbance power value (see Japanese Patent Application Laid-open No. 9-305235).
(c) A high frequency component is removed by allowing a current feedback to pass through a low-pass filter, thereby achieving a frequency component of a resonant frequency or lower. Based on a value of the current feedback which has passed through the low-pass filter, it is determined as to whether or not a collision occurs. In that case, two low-pass filters for a low speed and a high speed are prepared, so that the filter is switched according to a speed based on a speed command or a speed feedback (see Japanese Patent Application Laid-open No. 2001-150287).
(d) A quantity of a state of an actual machine unit including a motor, a mechanism unit to be driven by the motor and a motor control unit is compared with a quantity of a state of a simulation unit having a model of the motor, a model of the mechanism unit and a model of the motor control unit, so that the comparison result is compared with a set threshold value to detect a contact (see Japanese Patent Application Laid-open No. 2004-364396).
In the case where the collision is detected based on the estimated load torque estimated by the observer, the output torque from the servo motor is increased, and further, the estimated load torque is also increased when the servo motor is accelerated or decelerated. Therefore, an abnormal load determination level based on the estimated load torque has to be increased, thereby raising a problem of the difficulty in detecting a collision with accuracy. In order to solve the above-described problem, there is a known method in which a filter for smoothening a estimated disturbance load torque is provided by means of a dynamic/static friction correction or the like to suppress an increase in estimated load torque at the time of the acceleration or the deceleration.
When the disturbance load torque is estimated by the observer or the like, and then, the collision is detected based on the estimated disturbance load torque, it is desirable that the abnormal load determination level for use in determining the collision should be approximate to and greater than the estimated disturbance load torque generated during a normal operation as possible in order to detect the collision with a higher accuracy. However, since the output torque of the motor is increased and the estimated disturbance load torque is also increased at the time of the acceleration or the deceleration, the abnormal load determination level has to accord with the estimated disturbance load torque at the time of the normal acceleration or the normal deceleration: in contrast, the abnormal load determination level tends to be too high except for the time of the acceleration or the deceleration. Even if the estimated disturbance load torque is smoothened and suppressed by the filter at the time of the acceleration or the deceleration, the torque is fluctuated according to a lubrication state in the drive mechanism or the movable unit, thereby leading to the difficulty in producing a sufficient filter effect.